Method for wafer polishing and method for polishing-pad dressing

ABSTRACT

It is arranged such that the least common multiple of two numbers m and n of which one is prime to the other, is made as large as possible where the number m is the rotational speed (rpm) of a platen with a polishing pad affixed thereto and the number n is the rotational speed (rpm) of a carrier with a wafer mounted thereon. As a result of such arrangement, it is not until the platen completes m revolutions that a point on the polishing pad that comes into contact with a fixed point on the wafer returns to the original contact point with the fixed point at the start of polishing, and the resulting trajectory is therefore spread uniformly over the polishing pad. Each point on the wafer is brought into contact with most surface regions of the polishing pad, therefore preventing the wafer from undergoing deterioration in planarity uniformity due to a particular point on the wafer, on one hand, frequently coming into contact with low polishing-rate regions in the polishing pad and due to the other points on the wafer, on the other hand, less frequently coming into contact with the regions.

This application is a divisional of application Ser. No. 09/108,323filed Jul. 1, 1998 now Pat. No. 6,180,423.

BACKGROUND OF THE INVENTION

This invention relates to a method-for polishing a surface of a wafer byCMP (chemical-mechanical polishing) and to a method for dressing apolishing pad which is used in such CMP.

Chemical-mechanical polishing (CMP), a combination of chemical andmechanical polishing, is an attractive polishing process for planarizingwafers incorporating therein semiconductor integrated circuits, to suchan extent that the wafers are provided with almost perfect surfaceflatness. In a typical CMP technique, a wafer to be polished is mountedonto a polishing pad attached to a platen. The wafer is then rotated,during which a slurry-like abrasive liquid (dispersion of a colloidalsilica in a liquid) is supplied between the wafer and the polishing pad,to polish a surface of the wafer.

SOG (spin-on-glass) and etch back are known in the art as a process forplanarizing an upper surface of a film such as an interlayer dielectricfilm of a wafer. In the former process a wafer is spin-coated with aglass solution prepared by dissolution of glass in an organic solvent.In the latter process a film of photo resist is deposited on aninterlayer dielectric film and these films are thereafter subjected tosimultaneous etch back processing. The CMP process has the advantageover these two processes in that wafers can be planarized more perfectlybecause the CMP process combines both chemical polishing and mechanicalpolishing. However, the current technology of the CMP process is notsatisfactory. Achieving ideal planarity everywhere in a wafer is stilldifficult and there is yet room for improvement in the CMP process.Various approaches have been made with a view to improving theuniformity of in-wafer planarity.

One of the approaches is set forth in Japanese Patent Publication(KOKAI) No. 8-339979. This application describes a technique forsupporting a wafer lower surface with the aid of fluid, to improve thein-wafer planarity uniformity.

Another approach is described in Japanese Patent Publication (KOKAI) No.9-225812. This application provides means for maintaining the degree ofplanarity at an adequate level while performing a CMP process, toimprove the in-wafer planarity uniformity.

The following Preston equation is known and is generally used tocalculate the CMP polishing rate (Rpo).

Rpo=k*P*V,

where k is the Preston coefficient, P is the pressure, and V is thepolishing pad/wafer relative speed.

In order to improve in-plane uniformity of the polishing rate, based onthe Preston equation, equalization in time quadrature of V (thepolishing pad/wafer relative speed) at any points on the wafer isdesired. In other words, it has been determined from the Prestonequation that such equalization is achieved at an arbitrary point on thewafer to provide best in-plane polishing rate uniformity if both apolishing pad and a wafer rotate at the same speed.

However, CMP is a combination of chemical polishing and mechanicalpolishing, which makes, in actual process, variations in polishing statecomplicated. It is difficult to constantly place a polishing pad in anideal state during a period of polishing. As described in a paperreported in VLSI Multilevel Interconnection Conference (1997), pp.175-179, not every condition derived from the Preston equation yieldsbest in-plane polishing rate uniformity. Although some reasons why thebest conditions sometimes happen to differ from the Prestonequation-based conditions may be pointed out, no novel guidelines forimproving in-plane polishing rate uniformity are proposed in theforegoing paper.

For example, when mounting a polishing pad of closed-cell-foam typepolyurethane onto a platen, the inventors of the present inventionbelieve that the polishing rate varies for the following mechanicalreason.

The closed-cell-foam type polyurethane polishing pad, as illustrated inFIG. 10, has at its surface a great number of recess portions with adiameter in a range of 50-100 μm. The recess portions result from thebreaking of closed cells at the surface, and a slurry-like abrasive isheld in the recess portions. During polishing, the abrasive is suppliedbetween a polishing pad and a wafer little by little. If polishingdebris, formed as a result of polishing of the wafer and the pad, iscollected in a recess portion, or if recess-portion blocking occurslocally owing to the load of the wafer, polishing is not performed on a.portion of the wafer corresponding to such a recess portion filled withpolishing debris. Because of the foregoing, the polishing pad willundergo a local variation in polishing rate, resulting in a drop inoverall polishing rate. To cope with this problem, the surface of thepolishing pad is grounded with a dressing disk having abrasive particlessuch as diamond after the polishing pad has been used for a certainlength of time. This allows the entire polishing pad to becomere-activated, and there are formed new recess portions at the surface.However, to date, it is difficult to completely prevent clogged recessportions and deterioration in planarity between one dressing and thenext dressing.

The inventors of the present invention noted that the following pointssuggest that the foregoing in-pad local polishing rate variationadversely affects uniformity of the wafer planarity.

Specifically, after a polishing pad is subjected to a dressing process,it sometimes occurs that grains of diamond, dettached from a dressingdisk and then remaining on a polishing pad, produce in the wafer a deep,large scratch visible to even the naked eye. This scratch was observedand the observation result shows that the size of the scratch is largeand deep as compared with those of the diamond grain. The reason whysuch a large scratch is created may be explained as follows. A grain ofdiamond, cut into a wafer, passes through a fixed trajectory many timeswith pad/wafer relative rotational motion, as a result of which theoriginal micro scratch gradually develops until visible to the nakedeye.

To summarize, in the case there exists the foregoing non-uniformity ofpolishing rate in a polishing pad, if there is locally created a lowpolishing-rate portion in the polishing pad which frequently passesthrough a corresponding wafer region (in other words if a fixed point onthe wafer frequently passes through a specific region on the polishingpad) the variation in polishing rate of the polishing pad graduallypromotes deterioration in wafer planarity uniformity. However, therelationship between polishing pad rotation and wafer rotation has beenlittle considered in conventional CMP processing.

SUMMARY OF THE INVENTION

Based on the appreciation of the foregoing problems, the presentinvention was made. Apart from the problem of polishing-pad planarityand the problem of accuracy (e.g., parallelism between wafer andpolishing pad), a major object of the invention is therefore to providea method capable of improving uniformity in the wafer planarity byachieving uniform distribution of regions of a polishing pad that comeinto contact with each point of the wafer without ill effect (i.e.,non-uniformity in the polishing rate in the polishing pad).

The present invention provides a wafer polishing method comprising thesteps of:

(a) rotating a polishing pad affixed to a platen at a first rotationalspeed;

(b) supplying an abrasive material over a surface of said polishing pad;and

(c) pressing a wafer to be polished against said polishing pad surfacewhile at the same time rotating said wafer at a second rotational speed;

wherein the ratio of said first rotational speed to said secondrotational speed is controlled such that a trajectory, formed by pointson said polishing pad that come, in turn, into contact with a fixedpoint on said wafer, is distributed uniformly on said polishing pad.

One important aspect of the method of the present invention is that,during polishing, an arbitrary point on the wafer is brought intocontact with as many points on the polishing pad as possible. Such amethod eliminates a harmful influence due to a variation in localpolishing rate occurring in the polishing pad, therefore improving thepost-polishing uniformity of planarity of a wafer surface to bepolished.

A variation to the foregoing method can be made in which said rotationalspeed ratio is controlled such that said points on said polishing pad donot form a substantially fixed trajectory during polishing.

One important aspect of the foregoing variation is that every point onthe wafer is brought into contact with many regions on the polishingpad. Such arrangement eliminates a harmful influence due to a variationin local polishing rate occurring in the polishing pad, thereforeimproving the post-polishing uniformity of planarity of a wafer surfaceto be polished.

Another variation to the foregoing method can be made in which saidrotational speed ratio is controlled such that, when said ratio isexpressed using two natural numbers m and n of which one is prime to theother, the least common multiple of said numbers m and n is ten orbeyond.

One important aspect of the foregoing variation is as follows. When theplaten makes m revolutions, the wafer makes mn/m (=n) revolutions, and apoint on the polishing pad that comes into contact with the fixed pointon the wafer returns to its home position and then moves over a fixedtrajectory. When the least common multiple of the numbers m and n islarge, however, the overall length of the fixed trajectory extends.Corresponding to such an extension, the fixed trajectory passes througha greater number of regions on the polishing pad. This avoids asituation of an arbitrary point on the wafer coming into contact with alow polishing-rate region, that is locally created in the polishing pad,with considerable frequency as compared with other points on the wafer.This improves the post-polishing uniformity of planarity of a wafersurface to be polished.

Yet another variation to the foregoing method can be made in which saidrotational speed ratio is controlled to be an approximate irrationalnumber.

On important aspect of the foregoing variation is that, even when theratio of the platen rotational speed to the wafer rotational speed isapproximately expressed by a ratio represented by integers, the leastcommon multiple of these integers is considerably large. Accordingly,until the time when points on the polishing pad that come into contactwith a certain point on the wafer enter a fixed trajectory, the fixedtrajectory have passed through every region on the polishing pad.Additionally within a given polishing time that is practically limited,points on the polishing pad that come into contact with one point on thewafer will not get into a fixed trajectory. Accordingly, the foregoingoperation and effects of the present invention can be significantlyobtained.

Another variation to the foregoing method can be made in which saidpolishing pad is formed of a closed-cell-foam type polyurethane resin.

One important aspect of the foregoing variation is as follows. Even whena low polishing-rate portion is locally created because a recessportion, which is formed by breaking of a closed cell, becomes blockedwith polishing debris or is broken, a case, in which influence. by thepresence of such a low polishing-rate portion is strongly exerted ononly a specific point on the wafer, does not occur by virtue of theforegoing operation. This ensures that the foregoing operation andeffects of the present invention can be obtained.

In another variation to the foregoing method, said polishing pad isprovided with periodically-formed grooves or pores.

One important aspect of the foregoing variation is that smooth supplyand discharge of a slurry-like abrasive material are carried out throughgrooves or pores. Although a clogged groove or pore can be taken as aninactive region that does not substantially contribute to the action ofpolishing, a case, in which influence by the presence of such a cloggedgroove is strongly exerted on only a specific region, does not occur byvirtue of the foregoing operation. This ensures that the foregoingoperation and effects of the present invention can be obtained.

The present invention also discloses a polishing-pad dressing methodcomprising the steps of:

(a) rotating a polishing pad affixed to a rotary platen, at a firstrotational speed; and

(b) pressing a dresser against a surface of said polishing pad while atthe same time rotating said dresser at a second rotational speed foractivation of said polishing pad surface;

wherein the ratio of said first rotational speed to said secondrotational speed is controlled such that a trajectory, formed by pointson said polishing pad that come into contact with a fixed point on saiddresser, is distributed uniformly on said polishing pad.

A variation to the foregoing method can be made in which said rotationalspeed ratio is controlled such that said points on said polishing pad donot form a substantially fixed trajectory during polishing.

Another variation to the foregoing method can be made in which saidrotational speed ratio is controlled such that, when said ratio isexpressed using two natural numbers m and n of which one is prime to theother, the least common multiple of said numbers m and n is ten orbeyond.

Yet another variation to the foregoing method can be made in which saidrotational speed ratio is controlled to be an approximate irrationalnumber.

Important aspects of the foregoing variations to the aforesaid methodare as follows. A case is considered in which a polishing pad issubjected to dressing with the aid of a dresser with very fine diamondgrains embedded therein. If diamond grains that differ from one anotherin shape and in dimensions draw the same trajectories respectively onthe polishing pad, this results in deterioration in dressing uniformity.However, such deterioration can be suppressed by the present invention,resulting in providing almost uniform polishing pad activation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view outlining a way of polishing a wafer by aCMP polishing apparatus of a first embodiment of the invention.

FIG. 2 is a complex plane representation useful for understandingrelative motion between a polishing pad and a wafer in the firstembodiment.

FIG. 3 is a top view showing respective contact positions at which afixed point on the wafer contacts with the polishing pad when thepolishing pad makes revolutions an integral number of times, namely onerevolution, two revolutions, and m revolutions in the first embodiment.

FIG. 4 is a top view of a trajectory drawn on the polishing pad inExperimental Example 1.

FIG. 5 is a top view of a trajectory drawn on the polishing pad inExperimental Example 2.

FIG. 6 is a top view of a trajectory drawn on the polishing pad inComparative Example 1.

FIG. 7 is a top view of a trajectory drawn on the polishing pad inComparative Example 2.

FIG. 8 is a top view of a trajectory drawn on the polishing pad inComparative Example 3.

FIG. 9 is a diagram showing both variations in average polishing rateand variations in average polishing rate of different wafers prepared byperforming CMP processing under conditions of Experimental Example 2 andunder conditions of Comparative Example 3 respectively.

FIG. 10 shows in cross section a surface state of a polishing pad underpolishing by CMP.

FIG. 11 is a perspective view outlining a polishing pad dressing methodof a second embodiment of the invention.

FIG. 12 is a complex plane representation useful in understandingrelative motion between a polishing pad and a dresser in the secondembodiment.

FIG. 13 is a top view showing respective contact positions at which afixed point on the dresser contacts with the polishing pad when thepolishing pad makes revolutions an integral number of times, namely, onerevolution, two revolutions, and m revolutions in the second embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention are now described below.

Embodiment 1

FIG. 1 is a perspective view of an exemplary structure of a polishingapparatus used in a CMP process in accordance with a first embodiment ofthis invention. A CMP polishing apparatus of the present embodiment,manufactured by Speedfam Company Limited, includes a disk-like platen 1rotatable about its central axis, a platen shaft 2 which centrallysupports the platen 1, a polishing pad 3 affixed onto the platen 1 andformed of a closed-cell-foam type polyurethane resin and an unwovencloth, a disk-like carrier 4 on which is mounted a silicon wafer 6, acarrier shaft 5 which centrally supports the carrier 4, and a polishingliquid supply apparatus 7 for supplying a slurry-like polishing liquid 8the main component of which is colloidal silica. Both the platen shaft 2and the carrier shaft 5 are forcefully rotated by servomotor or thelike. The rotational speed of the platen shaft 2 and the rotationalspeed of the carrier shaft 5 are variably controlled independently ofeach other.

FIG. 2 is a complex plane representation showing a positionalrelationship between the polishing pad 3 and the wafer 6 in the CMPpolishing apparatus of the present invention. Making reference to FIG.2, the manner of relative motion resulting from the rotation of thepolishing pad 3 (i.e., the rotation of the platen 1) and the rotation ofthe wafer 6 (i.e., the rotation of the carrier 4), is illustrated.

Parameters of FIG. 2 concerning both the rotational motion of thecarrier 2 and the rotational motion of the platen 1 are as follows.

r: distance between wafer center P (i.e., the wafer rotational center)and point Z on the wafer

R: distance between P and polishing pad center O (i.e., the polishingpad rotational center)

w: wafer rotational angular speed

θ: wafer initial phase angle

wp: platen rotational angular speed

The carrier 4 can hold thereon a plurality of wafers in some cases.However, for convenience the present embodiment is illustrated in termsof a case in which only one wafer 6 is mounted on the carrier 4.Additionally, the center of the wafer 6 coincides with that of thecarrier 4 and the centre of the polishing pad 3 coincides with that ofthe platen 1.

Defining a state, in which the wafer center P and the polishing padcenter O are offset in y (imaginary) axis direction in a complex plane,as an initial state, and if the initial phase angle θ of the point Z onthe wafer and the polishing time t in a rest frame are variables, theposition of the point Z in rotational motion in clockwise (CCW)direction is given by the following equation (1). In the equation (1), irepresents the imaginary unit.

Z(t, θ)=i R+r Exp{i(−wt+θ)}  (1)

If such wafer motion is observed on the polishing pad which rotates inCCW direction, this determines a trajectory formed or drawn by the pointZ on the polishing pad. What is required to find the trajectory is tofind a CCW rotational mapping of Equation (1), which is given by thefollowing equation (2).

Z−Zp(t, θ)=Exp(i wp t)[i R+r Exp{i(−wt+θ)}]=i R Exp(i wp t)+rExp[i{(wp−w)t+θ}]  (2)

How the trajectory varies is determined by the ratio of w to wp on thebasis of Equation (2). For example, when wp is twice w, the followingequation (4) is derived from substitution of Equation (3) in Equation(2).

wp=2w  (3)

Zp=i R Exp(i 2wt)+r Exp{i(wt+θ)}  (4)

If the location of a point X on the wafer which is advanced in initialphase angle by pi (180°) with respect to the initial phase angle θ ofthe point Z is obtained from Equation (4), the result is given by thefollowing equation (5).

Zp(t, θ+pi)=i R Exp(i 2wt)+r Exp[{i (wt+θ+pi)}]  (5)

Further, if the location of the point X after a lapse of a time pi/w isobtained from Equation (4), this results in the following equation (6).Equations (4) and (6) completely agree with each other.

Zp(t+pi/w, θ+pi)=i R Exp{i(2wt+2pi)+r Exp[i {w(t+pi/w)+θ+pi)}]=i R Exp(i2wt)+r Exp[{i(wt+θ+pi)}]=Zp(t, θ)  (6)

In other words, if w/wp=½, a trajectory drawn on the pad by contactbetween the point Z and the pad completely agrees with another drawn onthe pad by a contact point between the point X and the pad (these twopoints Z and X are in symmetry with respect to P, in other words theydiffer from each other in phase by 180°), with only a time lag ofw/pi=(½ of the period of w).

A commonly-used condition of w/wp=½ (for example, w=30 rpm and w=60 rpm)means that both an arbitrary point on the wafer and another pointopposite thereto with respect to the wafer center keep rotating alongthe same trajectory thirty times per minute.

If a polishing pad, on which two different points on a wafer move alongthe same trajectory, has a local factor that contributes to a variationin polishing rate, such a micro factor (i.e., the local factor) willgradually develop on the surface of the wafer thereby finally producingan unwanted macro phenomenon that deteriorates planarity uniformity ofthe wafer. For instance, if a notch present on one-point causes apolishing pad surface to undergo a non-uniform variation therebyresulting in an abnormal polishing rate, this has an effect on theopposite side to the one point; for example, the polishing pad iscompressed excessively.

Generally, in a case in which the polishing pad angular speed wp is 2ntimes as large as the wafer angular speed w where the number n is anyinteger, points of the wafer (the phase difference in wafer rotationtherebetween being pi/n) move along the same trajectory over thepolishing pad.

For example, if wp:w=4:1, four points of the wafer, i.e., an arbitrarypoint, a second point (rotated 90° from the arbitrary point around thewafer center), a third point (rotated 180°), and a fourth point (rotated270°), move along the same trajectory. Such a situation must be avoidedin CMP recipe preparation. However, the relationship between platenrotational speed and carrier rotational speed has been little considereduntil the present invention.

Inconveniences occur, even when the platen-carrier rotational speedratio is simply expressed by integers, let alone when the ratio isexpressed in a simple ratio form such as (integer):1. An exemplary caseis now considered in which the ratio of the polishing pad angular speedand the wafer angular speed is m:n where the number m is prime to thenumber n and the numbers m and n are positive integers other than 1.FIG. 3 is a diagram showing rotational states of the polishing pad androtational states of the wafer when the polishing pad makes onerevolution, when the polishing pad makes two revolutions, and when thepolishing pad makes m revolutions respectively. Suppose here that PointZ on the wafer is in contact with Point S on the polishing pad in theinitial state (see FIG. 3). Point S is a start point at which a fixedtrajectory starts. When the platen makes one revolution from the initialstate, the carrier makes n/m revolutions. Since the number m is prime tothe number n, division of n by m, i.e., n/m, never produces an integralresult, and Point S′ on the polishing pad that comes into contact withPoint Z on the wafer will never conform to Point S. When the platenmakes m revolutions, the carrier makes mn/m (=n) revolutions, and PointS′ on the polishing pad that comes into contact with Point Z on thewafer conforms to Point S (i.e., the original contact point) for thefirst time, in other words a point on the polishing pad that comes intocontact with Point Z on the wafer arrives at Point. S where the fixedtrajectory starts. The polishing pad and the wafer thereafter repeat thesame relative motion, as a result of which Point Z on the wafer movesalong the fixed trajectory on the polishing pad.

When the least common multiple (L.C.M.) of the integers m and n (thenumber m is prime to the number n), i.e., mn, is large, the length ofthe fixed trajectory extends. In addition to making contact with aspecific point on the polishing pad, Point Z on the wafer is evenlybrought into contact also with many other points on the polishing pad.On the other hand, when the L.C.M. is small, Point Z soon returns toStart Point S and thereafter moves along the same trajectory, in otherwords Point Z comes into contact with limited regions. This produces thepossibility that a specific point on the wafer is frequently broughtinto contact with a low polishing-rate region that is locally created inthe polishing pad while other points on the wafer infrequently come intocontact with the region.

The following conclusions are drawn from the above consideration.

1. When the platen-carrier rotational speed ratio is expressed by twonatural numbers, i.e., n and m (the number n is prime to the number m),it is preferable to make the L.C.M. of these numbers m and n, i.e., mn,as large as possible. The experiments, which are described later, showthat the L.C.M. of the numbers m and n (mn) is preferably 10 or beyond.

2. It is particularly preferred that the platen-carrier rotational speedratio, i.e., the platen-carrier angular speed ratio, nearly correspondsto, or is approximated to an irrational number that cannot be expressedin the form of a natural (rational) number m/n. Practically it isdifficult to allow the ratio to exactly correspond to an irrationalnumber; however, if the ratio is an approximate irrational number thismakes it possible for Point Z on the wafer to come into contact withalmost every point on the polishing pad without travelling on the fixedtrajectory on the polishing pad in a limited polishing period of time.Setting of such a rotational speed ratio which is an approximateirrational number can be achieved easily by setting rotational speedsfor motors that drive the carrier shaft 5 and the platen shaft 2.

3. Taking into account a practically limited period of time taken forpolishing, it is sufficient that the platen-carrier rotational speedratio is set in such a way as to prevent entrance of a point on thepolishing pad to a fixed trajectory during polishing. In other words therotational speed ratio is set in order for a point on the polishing padthat comes into contact with Point Z on the wafer not to conform toStart Point S, with the polishing pad and the wafer rotated an integralnumber of times.

4. Taking into account the state of the foregoing trajectory, it ispreferred that, although the surface region of the polishing pad isdivided into sub-regions by a trajectory drawn by a point on thepolishing pad that comes into contact with Point Z on the wafer (seeFIG. 5), these sub-regions are uniform in size and fine withinconcentric ring-like regions, i.e., within ring-like regions havingidentical radii from the polishing pad center O. In other words, it ispreferred that the foregoing trajectory is evenly distributed on thepolishing pad, to densely form lattice-like patterns thereon.

Substantially, the number of revolutions of a CMP polishing apparatus ism (rpm) where m is an integer. Points on the polishing pad that comeinto contact with Point Z on the wafer repeatedly form the sametrajectory at polishing time intervals of about one minute. The processof polishing is carried out for about four minutes at most, and it istherefore preferred that the same fixed trajectory is not drawn fourtimes or more during polishing.

In the above-described analyses the carrier (wafer) is subjected torotational motion only. The carrier may be subjected, in addition torotational movement, to reciprocating motion (translational motion)either in a direction perpendicular to the direction in which the platenrotates, in a direction corresponding to the direction in which theplaten rotates, or in a direction diagonally intersecting with thedirection in which the platen rotates. Additionally, by sufficientlyexpanding the range of such translational motion to replace the numberof times per unit time reciprocate motion is carried out with the numberof revolutions per unit time of the carrier, the same effects as theabove can be achieved without having to rotate the carrier depending onthe case.

In the present embodiment, the number of wafers mounted on the carrieris one. The present invention is also applicable to cases in which aplurality of wafers are mounted on a single carrier. In such a case eachwafer point rotates on the carrier center and the analyses of theforegoing equations (1) to (6) can be utilized, by taking (i) r=thedistance from the carrier center to Point Z of the wafer and (ii) R(offset distance)=the distance from the polishing pad center O to thecarrier center.

Experimental examples and comparative examples (conventional conditions)for analyzing trajectories on polishing pads (platens) are now describedbelow. A polishing apparatus by Speedfam Company Limited was used. Ineach example, the initial phase angle θ is 45 degrees, the waferdiameter r is 100 mm, the offset distance R is 162.5 mm, and thepolishing time is 60 seconds. A target for polishing is a p-type TEOSfilm or a p-type BPSG film formed on a wafer.

EXPERIMENTAL EXAMPLE 1

FIG. 4 illustrates a trajectory drawn on a polishing pad by a singlepoint (a fixed point) on a wafer in Experimental Example 1 in accordancewith the present invention. Polishing conditions of the presentexperimental example are CMP conditions for planarization of BPSG filmsand the platen rotational speed (wp) is 23 rpm. The carrier rotationalspeed (w) is 17 rpm. In other words, the platen-carrier rotational speedratio is expressed by two numbers, 23 and 17, of which one number isprime to the other number, and the L.C.M of these two numbers, i.e.,23×17 (=391), is large. As can be seen from the figure, within a timeless than the polishing time (60 seconds) there is drawn no fixedtrajectory on the polishing pad and a certain point on the wafer doesnot move along a fixed trajectory on the polishing pad. When polishingis carried out for 60 seconds, i.e., when the platen makes 23revolutions, the fixed point on the wafer returns to the originalcontact point on the polishing pad, in other words a point on thepolishing pad that comes into contact with the fixed point on the waferdoes not enter the fixed trajectory. Even in such a case the fixed pointon the wafer equally comes into contact with many regions on thepolishing pad, therefore avoiding ill effects due to a local variationin polishing rate.

EXPERIMENTAL EXAMPLE 2

FIG. 5 is a diagram showing a trajectory drawn on a polishing pad by afixed point on a wafer in Experimental Example 2 in accordance with thepresent invention. Polishing conditions of the present experimentalexample are CMP conditions for planarization of p-type TEOS films. Theplaten rotational speed (wp) is 61 rpm and the carrier rotational speed(w) is 43 rpm. In other words, the platen-carrier rotational speed ratiois expressed by two numbers, 61 and 43, of which one number is prime tothe other number, and the L.C.M of these two numbers, i.e., 61×43(=2643), is large. As can be seen from the figure, within a time lessthan the polishing time (60 seconds), a point on the polishing pad thatcomes into contact with the fixed point on the wafer does not enter afixed trajectory. When polishing is carried out for 60 seconds, i.e.,when the platen makes 61 revolutions, a certain point on the waferreturns to the original contact point on the polishing pad. Even in sucha case, the point on the wafer equally comes into contact with manyregions on the polishing pad, therefore avoiding ill effects due to alocal variation in polishing rate.

COMPARATIVE EXAMPLE 1

FIG. 6 is a diagram of a trajectory drawn on a polishing pad by a pointon a wafer in Comparative Example 1. Polishing conditions of ComparativeExample 1 are conventional CMP conditions used for planarization of BPSGfilms. The platen rotational speed wp is 20 rpm and the carrierrotational speed w is 20 rpm. As can bee seen from FIG. 6, resonanceoccurs so that a fixed trajectory, which is in an almost perfectcircular form, is drawn on the polishing pad. This shows that a certainpoint on the wafer frequently comes into contact with limited regions onthe polishing pad.

COMPARATIVE EXAMPLE 2

FIG. 7 is a diagram of a trajectory drawn on a polishing pad by a pointon a wafer in Comparative Example 2. Polishing conditions of ComparativeExample 2 are conventional CMP conditions used for planarization ofp-type TEOS films. The platen rotational speed wp is 60 rpm and thecarrier rotational speed w is 30 rpm. In other words, the platen-carrierrotational speed ratio is expressed by two numbers, 2 and 1, of whichone number is prime to the other number, and the L.C.M of these twonumbers, i.e., 2×1 (=2), is small. As shown in the figure, a point onthe polishing pad that comes into contact with a fixed point on thewafer returns to the start point of a fixed trajectory every time theplaten makes two revolutions and thereafter resonance occurs resultingin motion along the fixed trajectory. This shows that a certain point onthe wafer frequently (but less frequently than in Comparative Example 1)comes into contact with only limited regions on the polishing pad.

COMPARATIVE EXAMPLE 3

FIG. 8 is a diagram of a trajectory drawn on a polishing pad by a fixedpoint on a wafer in Comparative Example 3. Polishing conditions ofComparative Example 3 are conventional CMP conditions. The platenrotational speed wp is 60 rpm and the carrier rotational speed w is 40rpm. In other words, the platen-carrier rotational speed ratio isexpressed by two numbers, 3 and 2, of which one number is prime to theother number, and the L.C.M of these two numbers, i.e., 3×2 (=6), issmall. As shown in the figure, a point on the polishing pad that comesinto contact with the fixed point on the wafer returns to the startpoint of a fixed trajectory every time the platen makes threerevolutions and thereafter resonance occurs resulting in motion alongthe fixed trajectory. This shows that a certain point on the waferfrequently (but less frequently than in Comparative Examples 1 and 2)comes into contact with only limited regions on the polishing pad.

Next, the difference in uniformity of the wafer planarity between theExperimental Examples polishing conditions utilizing the presentinvention and the Comparative Examples polishing conditions is describedbelow. FIG. 9 graphically compares Experimental Example 2 (wp=61 rpm;w=43) and Comparative Example 3 (wp=60 rpm; w=40 rpm). In other words,SSR (nm/min) indicated by ▪ and WIWNU (Within-Wafer-Non-Uniformity) (%)indicated by 58 of Experimental Example 2 are compared with polishingrates indicated by  and polishing rate variations indicated by ∘ ofComparative Example 3, where SSR indicates the average wafer polishingrate and WIWNU indicates the wafer in-plane polishing rate variation. Aload of 140 KgG was applied to the carrier, and wafer polishing ratemeasurement is carried out at several locations, exclusive of regionslaying within 5 mm from the periphery of the wafer. A variation inpolishing rate is expressed by a value obtained as a result of divisionof a difference between the maximum and minimum of measured values atlocations in the same wafer by an average measured value. FIG. 9 showsthat there is no difference in average polishing rate betweenComparative Example 3 and Experimental Example 2; however, if CMPprocessing is performed using Experimental Example's 2 conditions inaccordance with the present invention, this achieves further reductionsin in-wafer polishing rate variation as compared with ComparativeExample 3. Uniformity of the wafer planarity is clearly improved byutilizing the present invention.

It follows from the foregoing experimental and comparative examplesthat, when the ratio of wp (the platen rotational speed) to w (the waferrotational speed) is expressed by natural numbers m and n of which oneis prime to the other, the L.C.M. of these two numbers is preferably 10or beyond in order to obtain the present invention's benefits.

In such a case it is much preferred that none of the numbers m and n are1 (for example, m=5 and n=2), which is however not absolutely necessary.For instance, in case m is assigned a value of 10 and n a value of 1,such a value setting allows the carrier to make one revolution when theplaten makes ten revolutions, and, at this point in time, a contactpoint with a fixed point on the wafer returns to the start point of afixed trajectory on the polishing pad. In other words, a fixedtrajectory formed on the polishing pad comprises a first spiral thatgets to an internal-diameter portion from an external-diameter portionwhen it makes five revolutions and a second spiral which rotates in adirection opposite to that of the first spiral and which gets to anouter peripheral portion from an inter peripheral portion when it makesfive revolutions. On the other hand, if m and n are assigned a value of1 and a value of 10 respectively (m=1 and n=10), this value settingallows the platen to make one revolution when the carrier makes tenrevolutions, and, at this point in time, a contact point returns to thestart point of a fixed trajectory. The resulting trajectory is in theform of a small coil of ten rounds. In both of the foregoing settings(i.e., the L.C.M. of the numbers m and n is ten or greater), a fixedtrajectory is formed on the polishing pad in order that it can passthrough many regions, therefore avoiding circumstances in which there isan increase in the probability that a specified point on the waferfrequently comes into contact with regions that are low in polishingrate as compared with other points.

Embodiment 2

The present invention can be applied to a process, i.e., a dressingprocess step, for dressing of a polishing pad with a diamond dresser tomake polishing-pad activation (polishing rate recovery). Grains ofdiamond, which are very fine, are embedded into a surface of a diamonddresser. These diamond grains have different shapes and dimensions. Ifeach diamond grain draws the same trajectory with considerable frequencyat the time of performing a dressing on a polishing pad that is beingrotated while rotating a diamond dresser, this becomes a bar toobtaining the uniformity of dressing, as in the case of polishing. As aresult, polishing-pad activation is not carried out in uniform fashion,in addition to which local variations in polishing rate occur in thepolishing pad. The foregoing first embodiment can be applied intact todressing uniformity processing.

A second embodiment of the invention is now described. The secondembodiment provides a method for dressing of a polishing pad which usesthe first embodiment of the present invention.

FIG. 11 is a perspective view of a polishing apparatus when a dressingaccording to the second embodiment is performed on a polishing pad. ACMP polishing apparatus, used in the present embodiment as well as inthe first embodiment, is illustrated by reference to FIG. 11. The CMPpolishing apparatus, on one hand, includes a disk-like platen 1rotatable about its central axis, a platen shaft 2 which centrallysupports the platen 1, a polishing pad 3 affixed onto the platen 1 andformed of a closed-cell-foam type polyurethane 1 resin and an unwovencloth. A dresser apparatus, on the other hand, includes a disk-likedresser 11 and a dresser shaft 12 which centrally supports the dresser11. Both the platen shaft 2 and the dresser shaft 12 are forcefullyrotated by servomotor or the like. The rotational speed of the platenshaft 2 and the rotational speed of the dresser shaft 12 are variablycontrolled independently of each other.

FIG. 12 is a complex plane representation useful in understanding thepositional relationship between the polishing pad 3 in the polishingapparatus and the dresser 11. As can be seen from FIG. 12, the manner ofrelative motion between the rotation of the platen 1 and the rotation ofthe dresser 11 is basically identical with the manner of relative motionbetween the rotation of the platen 1 and the rotation of the carrier 4in the first embodiment of the present invention.

Parameters of FIG. 2 concerning both the polishing-pad rotational motionand the dresser rotational motion are as follows. The same signs as thefirst embodiment are used.

r: distance between dresser center P and point Z on the dresser

R: distance between P and polishing pad center O

w: dresser rotational angular speed

θ: dresser initial phase angle

wp: platen rotational angular speed

Equations (1)-(6) of the first embodiment are applicable to the secondembodiment without having to make any changes thereto.

Thus, if w/wp=½, a trajectory drawn on the pad by contact between thepoint Z and the pad completely coincides with another drawn on the padby contact between the point X and the pad (these two points Z and X arein symmetry with respect to P, in other words they differ from eachother in phase by 180°), with only a time lag of w/pi (½ of the periodof w).

Inconveniences occur when the platen-dresser rotational speed ratio issimply expressed by integers and when the ratio is expressed in a simpleratio form such as (integer):1. An exemplary case is now considered inwhich the ratio of the polishing-pad angular speed and the dresserangular speed is m:n where the number m is prime to the number n and thenumbers m and n are positive integers, exclusive of 1. FIG. 13 is adiagram showing rotational states of the polishing pad and rotationalstates of the dresser when the polishing pad makes one revolution, whenthe polishing pad makes two revolutions, and when the polishing padmakes m revolutions respectively. Let us assume here that Point Z on thedresser is in contact with Point S on the polishing pad in the initialstate (see FIG. 13). Point S is a start point at which a fixedtrajectory starts. When the platen makes one revolution from the initialstate, the dresser makes n/m revolutions. Since the number m is prime tothe number n, division of n by m, i.e., n/m, never yields an integralquotient, and Point S′ on the polishing pad that comes-into contact withPoint Z on the dresser will never conform to Point S. When the platenmakes m revolutions, the dresser makes mn/m (=n) revolutions, and PointS′ on the polishing pad that comes into contact with Point Z on thedresser now conforms to Point S (i.e., the original contact point) forthe first time, in other words a point on the polishing pad that comesinto contact with Point Z on the dresser arrives at Start Point S (thefixed trajectory start point). The polishing. pad and the waferthereafter repeat the same relative motion, as a result of which Point Zon the on the dresser moves along the fixed trajectory on the polishingpad.

When the least common multiple (L.C.M.) of the integers m and n (thenumber m is prime to the number n), i.e., mn, is large, the length ofthe fixed trajectory extends. In addition to contact with a specificpoint on the polishing pad, Point Z on the dresser is evenly broughtinto contact also with many other points on the polishing pad. On theother hand, when the L.C.M. is small, Point Z soon returns to StartPoint S and thereafter moves along the same trajectory, in other wordsPoint Z comes into contact with limited regions. This produces thepossibility that a specific point on the dresser is brought, withconsiderable frequency, into contact with a low polishing-rate regionthat is locally created in the polishing pad while other points on thedresser infrequently come into contact with the region in question.

By making a change in platen-dresser angular speed ratio in the presentembodiment, trajectories similar to FIGS. 4-8 are drawn or formed onpolishing pads respectively, as in the first embodiment.

The following conclusions are drawn from the above consideration.

1. When the platen-dresser rotational speed ratio is expressed by twonatural numbers, i.e., n and m (the number n is prime to the number m),it is preferred to make the L.C.M. of these numbers m and n, i.e., mn,as large as possible, preferably 10 or beyond.

2. It is particularly preferred that the platen-dresser rotational speedratio, i.e., the platen-dresser angular speed ratio, nearly correspondsto, or is approximated to an irrational number that cannot be expressedin the form of a natural (rational) number m/n.

3. It is sufficient that the platen-dresser rotational speed ratio isset in such a way as to prevent entrance of a point on the polishing padto a fixed trajectory during polishing. In other words, the ratio is setin order for a point on the polishing pad that comes into contact withPoint Z on the dresser not to conform to Start Point S with thepolishing pad and the dresser rotated an integral number of times.

4. It is preferred that, although the surface region of the polishingpad is divided into sub-regions by a trajectory drawn by a point on thepolishing pad that comes into contact with Point Z on the dresser (seeFIG. 5), these sub-regions are uniform in size and fine within aconcentric ring-like region, i.e., within a ring-like region havingidentical radii from the polishing pad center O. In other words, it ispreferred that the foregoing trajectory is evenly distributed on thepolishing pad, to densely form lattice-like patterns thereon.

Dressing is not a process step belonging in the manufacture ofsemiconductor devices (a step for dresser surface planarization), and itis therefore required that polishing rate activation be completed asquickly as possible. The present invention is available to realizingpolishing pad surface activation in a short time.

Other Embodiments

In the foregoing embodiments of the invention, it is arranged such thatthe direction of rotation of the platen and the direction of rotation ofthe carrier (dresser) are the same. It is to be noted that the inventionis not limited to such embodiments. For instance, even when they rotatein opposite directions, the same effects that the first and secondembodiments achieve can be obtained by, for example, arrangement thatthe platen-carrier (dresser) rotational speed ratio is expressed by twonatural numbers of which one is prime to the other and the L.C.M. ofthese numbers is 10 or greater.

In a variation to the foregoing, a polishing pad (formed ofclosed-cell-foam type polyurethane or the like material) is employed,having a series of about 1-mm grooves or pores formed, at a pitch in therange of from about 5 mm to about 10 mm, on a surface thereof. Thesegrooves (pores) are provided for smooth supply and discharge of aslurry-like abrasive liquid. Although, when local groove (pore) blockingoccurs to cause a certain groove to become blocked, such a cloggedgroove can be taken as an inactive region that does not contribute tothe action of polishing, even in such a case it is possible to improveuniformity of the in-wafer planarity by utilizing the present invention.

What is claimed is:
 1. A polishing-pad dressing method comprising thesteps of: (a) rotating a polishing pad affixed to a rotary platen, at afirst rotational speed; and (b) pressing a dresser against a surface ofsaid polishing pad while at the same time rotating said dresser at asecond rotational speed for activation of said polishing pad surface;wherein the ratio of said first rotational speed to said secondrotational speed is controlled such that a trajectory, formed by pointson said polishing pad that come into contact with a fixed point on saiddresser, is distributed uniformly on said polishing pad.
 2. Thepolishing-pad dressing method of claim 1 wherein said rotational speedratio is controlled such that said points on said polishing pad do notform a substantially fixed trajectory during polishing.
 3. Thepolishing-pad dressing method of claim 1 wherein said rotational speedratio is controlled such that, when said ratio is expressed using twonatural numbers m and n of which one is prime to the other, the leastcommon multiple of said numbers m and n is ten or beyond.
 4. Thepolishing-pad dressing method of claim 1 wherein said rotational speedratio is controlled to be an approximate irrational number.